Aqueous solution of zinc nitrite and method for production thereof

ABSTRACT

An aqueous zinc nitrite solution which contains substantially no calcium (Ca) ions is provided, in which, in terms of the aqueous zinc nitrite [Zn(NO 2 ) 2 ] solution having an NO 2  concentration of 10% by weight, the sodium (Na) ion concentration is 200 to 2000 ppm and the sulfate (SO 4 ) ion concentration is 20 ppm or less in the solution. The aqueous zinc nitrite solution can be prepared by providing a zinc compound and an alkali nitrite as raw materials and subjecting the raw materials to electrolytic synthesis through a double decomposition reaction using an ion-exchange membrane as a diaphragm. As the aqueous zinc nitrite solution is provided, an extremely efficient metal surface treatment is made possible which has a reduced amount of sodium ions. In particular, it contains substantially no sulfate ions and no calcium ions.

TECHNICAL FIELD

The present invention relates to an aqueous zinc nitrite solution and amethod for preparing the same. More particularly, the present inventionrelates to: an aqueous zinc nitrite solution making extremely efficientmetal surface treatment possible, where the amount of sodium ions isreduced and in particular a sulfate ions and calcium ions aresubstantially not present, making, in particular, a closed system offilm-forming treatment on metal possible; and a method for preparing thesame.

BACKGROUND ART

Zinc nitrite is known to decompose at around 100° C. with release ofnitrogen oxide when slowly heated in air. Also, zinc nitrite is known tobe soluble in water and vulnerable to hydrolysis and forms zincoxynitrite [ZnO.Zn(NO₂)₂] upon evaporation of its aqueous solution.

Generally, such zinc nitrite is prepared by a method of mixing zincsulfate with an ethanol solution of sodium nitrite, filtering theresulting precipitate, and evaporating and concentrating the filtrate toobtain crystals (see “Kagaku Daijiten” published on Mar. 15, 1984 byKYORITSU SHUPPAN CO., LTD.).

However, this method, which is a method in which sodium sulfate isseparated and removed by utilizing the difference in solubility betweenzinc nitrite and sodium sulfate, requires the use of ethanol, and theoperations of evaporation and concentration. The method has problems inthat it inevitably causes an increase in cost, sodium ions are alsoallowed to remain, and the like.

Therefore, it is currently difficult to obtain high purity zinc nitriteor its aqueous solution on an industrial scale.

Further, as a pretreatment process prior to painting of a metal surface,a series of steps of degreasing, washing with water, film formingtreatment, washing with water, and drying is generally included. As anexample of the film forming treatment, a treating method for forming afilm of zinc phosphate on the surface of steel is commonly adopted. As afilm-forming agent used for this purpose, a treating liquid prepared bydissolving zinc in phosphoric acid and diluting the resulting solutionwith water has been used. This treatment is termed “metal surfacetreatment”.

Furthermore, in order to promote the film forming reaction of metal, achemical such as sodium nitrite or sodium chlorate is added to thefilm-forming agent. These chemicals are called “promoters”. Addition ofthese promoters shows an effect that the forming treatment can beperformed at a lower temperature, and the film forming treatment time isreduced.

However, conventional sodium salts such as sodium nitrite and sodiumchlorate have a problem in that long usage of a treating bath increasesthe concentration of Na ions and as a result the pH of the treating bathis elevated so that components of the formed film precipitate in thetreating bath. Also, there is a problem in that, when recovering andregenerating the old treating liquid, accumulation of Na ions in thetreating bath destroys the balance of the bath so that removal of Naions from the recovered treating liquid is necessary. Usually, treatingliquid containing Na ions must be disposed of as industrial waste.

Further, the issue of environmental protection has recently attractedmuch attention even in the field of metal surface treating liquids, andattempts are being made to establish a closed system for treating baths.

On this account, more intensive studies than ever before are being madeof a metal surface-treating method with a lower generation of sludge.

The present inventors proposed an aqueous zinc nitrite solutiondistinguished as a film-forming promoter for a metal surface treatment,which contains substantially no sodium ions and no sulfate ions, and isobtained by reacting between zinc sulfate and calcium nitrite at firstand then conducting purification (Japanese Patent Application No.2000-141893). In addition, it is known that the presence of calcium ionsin the film-forming promoter causes the promoter to become sludge ascalcium phosphate in the surface-treating solution, for example, at thetime of mixing with a zinc phosphate film-forming solution. Usually, thesludge is periodically recovered to be prevented from accumulating in atreating bath. However, the operation of collecting the sludge is alsocomplicated, thereby failing to show an industrial advantage.

The present inventors have been dedicated to making repetitive studiesof an aqueous zinc nitrite solution useful as a film-forming promoterfor a metal surface treatment, and as a result discovered a nitriteaqueous solution having a reduced amount of sodium ions, in particular,containing substantially no sulfate ions and no calcium ions.

In other words, an object of the present is to provide an aqueous zincnitrite solution that allows extremely efficient metal surface treatmentand has a reduced amount of sodium ions, in particular, containingsubstantially no sulfate ions and no calcium ions, and a method forpreparing the same.

DISCLOSURE OF THE INVENTION

Under such circumstances, the present invention has been accomplished onthe basis of the knowledge that the generation of sludge is reduced andextremely efficient metal surface treatment is made possible by using anaqueous zinc nitrite solution as a film-forming promoter for the metalsurface treatment, in which the aqueous zinc nitrite solution containingsubstantially no sulfate ions and no calcium ions while containing 500to 2000 ppm of sodium ions is obtained by providing a zinc compound andan alkali nitrite as raw materials and subjecting the raw materials toelectrolytic synthesis through a double decomposition reaction using anion-exchange membrane as a diaphragm.

Thus, according to a first aspect of the present invention, there isprovided an aqueous zinc nitrite solution which contains substantiallyno calcium (Ca) ions, characterized in that, in terms of the aqueouszinc nitrite [Zn(NO₂)₂] solution having an NO₂ concentration of 10% byweight, sodium (Na) ion concentration is 200 to 2000 ppm and sulfate(SO₄) ion concentration is 20 ppm or less in the solution.

Also, according to a second aspect of the present invention, there isprovided a method for preparing an aqueous zinc nitrite solutionaccording to claim 1, characterized by including synthesizing a zinccompound and an alkali nitrite as raw materials by a doubledecomposition reaction using an ion-change membrane as a diaphragm.

It is preferable that the reaction is performed in an electrodialysiscell having a unit including one concentration chamber and two desaltingchambers sandwiching the concentration chamber which are formed by analternate arrangement of cation-exchange membranes and anion-exchangemembranes between an anode and a cathode.

Also, it is preferable that, in the method, an aqueous zinc compoundsolution is supplied to one of the desalting chambers and an aqueousalkali nitrite solution is supplied to the other of the desaltingchambers, and zinc ions are introduced through a cation-exchangemembrane and nitrite ions are introduced through an anion-exchangemembrane into a concentration chamber sandwiched between the desaltingchambers, to thereby obtain the objective aqueous zinc nitrite solution.

Also, according to a third aspect of the present invention there isprovided an aqueous zinc nitrite solution further including astabilizing agent.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic view showing the electrodialysis cell used inthe method for preparing an aqueous zinc nitrite solution of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

The aqueous zinc nitrite solution of the present invention is a solutionthat includes a component represented by the general formula Zn(NO₂)₂and H₂O mixed at any optional ratio.

In addition, even though industrial refining of the product obtained bythe conventional manufacturing process is accompanied by manydifficulties, the aqueous zinc nitrite solution of the present inventionis characterized in that the concentration of sodium (Na) ions isreduced, in particular, substantially no sulfate ions (SO₄) and nocalcium (Ca) ions are contained.

Here, the concentration of zinc nitrite in the aqueous solution isobtained by measuring the concentration of zinc (Zn) ions and theconcentration of nitrite ions, and provided as the concentration ofZn(NO₂)₂. The concentration of zinc ions is obtained by ICP luminescencespectrometry and the concentration of nitrite (NO₂) ions is obtained byion chromatography.

Furthermore, sodium (Na) ions, sulfate (SO₄) ions, and calcium (Ca) ionsare all measured by ICP luminescence spectrometry. Here, sulfate (SO₄)ions are measured as sulfur (S) and converted to sulfate ions.

Furthermore, the concentrations of sodium (Na) ions, sulfate (SO₄) ions,and calcium (Ca) ions in the present invention are calculated byconverting into 10% by weight in terms of NO₂. The concentration ofsodium ions is 200 to 2,000 ppm, preferably 500 to 1500 ppm. Theconcentration of sulfate ions is 20 ppm or less, preferably 10 ppm orless. In the present invention, furthermore, the expression “containssubstantially no calcium (Ca) ions” means that the concentration ofcalcium (Ca) ions is 100 ppm or less, preferably 10 ppm or less.

In the aqueous zinc nitrite solution of the present invention, theconcentration of nitrite ions is 5 to 15% by weight, preferably 9 to 12%by weight, the concentration of Zn ions is 5 to 10% by weight,preferably 7 to 9% by weight, and the concentration of Zn(NO₂)₂ is 10 to25% by weight, preferably 15 to 20% by weight.

In the aqueous zinc nitrite solution of the present invention, sodiumions, in particular, sulfate (SO₄) ions and calcium (Ca) ions arereduced in concentration. Therefore, using this in the composition of afilm-forming promoter for a metal surface treatment allows the designingof an extremely efficient surface treatment system.

Next, the method for preparing the aqueous zinc nitrite solution of thepresent invention will be described.

The method for preparing the aqueous zinc nitrite solution of thepresent invention is characterized in that a zinc compound and an alkalinitrite are, provided as raw materials and subjected to electrolyticsynthesis through a double decomposition reaction using an ion-exchangemembrane as a diaphragm in the aqueous solution.

The present invention is preferably carried out as follows. That is, inan electrodialysis cell provided with a unit composed of a concentrationchamber and two desalting chambers sandwiching the concentration chamberformed by the alternate arrangement of cation-exchange membranes andanion-exchange membranes between the anode and the cathode. Each of thedesalting chambers is composed of an anion-exchange membrane on an anodeside and a cation-exchange membrane on a cathode side. An aqueous zinccompound solution as a raw material is supplied to the desalting chamberon the anode side and an aqueous alkali nitrite solution is supplied tothe desalting chamber on the cathode side. An electric current isapplied to introduce zinc ions through a cation-exchange membrane andnitrite ions through an anion-exchange membrane into a concentrationchamber sandwiched between two desalting chambers. Therefore, theobjective aqueous zinc nitrite solution is obtained in the concentrationchamber.

The aqueous zinc compound solution is an aqueous solution prepared bydissolving a water-soluble zinc compound in water. The zinc compoundincludes, for example, zinc sulfate, zinc nitrate, and zinc chloride,and such compounds may be used singly or in combinations of two or more.Among them, zinc sulfate is preferred from the standpoint of commercialavailability and low price.

The concentration of the aqueous zinc compound solution is notparticularly restricted but is preferably not higher than the saturationconcentration at room temperature, and specifically 0.5 to 2.0 mol/L,still more preferably 0.9 to 1.3 mol/L.

The aqueous alkali nitrite solution, another starting raw material, isan aqueous solution prepared by dissolving a water-soluble alkalinitrite in water. The alkali nitrite includes, for example, sodiumnitrite, potassium nitrite, and lithium nitrite, and these may be usedsingly or in combinations of two or more. Among them, sodium nitrite ispreferred from the standpoint of commercial availability and low price.

The concentration of the aqueous solution of a soluble alkali nitrite isnot particularly restricted but is preferably not higher than thesaturation concentration at room temperature, and specifically 1.5 to6.0 mol/L, still more preferably 3.0 to 4.5 mol/L.

The cation-exchange membrane that can be used in the present inventionis not particularly restricted. For example, Selemion CMV (product ofAsahi Glass Co.), Neocepta CM-1 (product of Tokuyama Co.), Nafion 324(product of DuPont), and the like may be given. on the other hand, theanion-exchange membrane is not particularly restricted and may includeSelemion AMV (product of Asahi Glass Co.), Neocepta AM-1 (product ofTokuyama Co.), and the like.

The anode and cathode for use in the electrodialysis cell are each madeof a suitable material in a suitable shape depending on the material andan electrodialysis cell configuration, and more specifically, include ametallic material such as platinum, iron, copper, or lead or acarbonaceous material.

The reaction temperature is 10 to 50° C., preferably 20 to 40° C. Thecurrent density is 1.0 A/dm³ to limiting current density, preferably 1.5to 5.0 A/dm³. The current time is 5 to 50 hours, preferably 10 to 40hours, but is not necessarily restricted to those conditions.

Further, detailed description will be made of the case where the methodfor preparing an aqueous zinc nitrile solution is carried out using theelectrodialysis cell shown in the figure.

In the electrodialysis cell shown in the figure, from the anode side tothe cathode side, an anion-exchange membrane (A1), a cation-exchangemembrane (C1), an anion-exchange membrane (A2), and a cation-exchangemembrane (C2) are arranged in order. In addition, it is provided with ananode chamber/a desalting chamber (I)/a concentration chamber (II)/adesalting chamber (III)/a cathode chamber in that order.

The anode chamber and the cathode chamber are supplied with anelectrolyte such as Na₂SO₄, NaCl, or NH₄Br. Also, the desalting chamber(I) is supplied with the above aqueous zinc compound solution. On theother hand, the desalting chamber (III) is supplied with the aboveaqueous alkali nitrite solution. By applying the current, an aqueouszinc nitrite solution is produced in the concentration chamber (II).

The concentration of the aqueous zinc nitrite solution obtained in theconcentration chamber (II) rises as the current time is extended, butthe sodium ion and sulfate ion concentrations of the solution in termsof an aqueous zinc nitrite [Zn(NO₂)₂] solution having an NO₂concentration of 10% by weight also tend to rise. Therefore, it ispreferable to control the current time such that the sulfate temperatureion concentration will be 20 ppm or less and the sodium ionconcentration will be 2000 ppm or less.

In the aqueous zinc nitrite solution of the present invention thusobtained, the concentration of nitrite ions is 5 to 15% by weight,preferably 9 to 12% by weight, the concentration of Zn ions is 5 to 10%by weight, preferably 7 to 9% by weight, and the concentration ofZn(NO₂)₂ is 10 to 25% by weight, preferably 15 to 20% by weight.

In addition, in terms of an aqueous zinc nitrite [Zn(NO₂)₂] solutionhaving an NO₂ concentration of 10% by weight, the sodium concentrationis 200 to 2000 ppm, preferably 500 to 1500 ppm, the sulfate ionconcentration is 20 ppm or less, preferably 10 ppm or less, and theconcentration of calcium is 100 ppm or less, preferably 10 or less.Therefore, the aqueous zinc nitrite solution of the present inventioncan be used as the composition of the film-forming promoter for metalsurface treatment.

Furthermore, according to the method for preparing the aqueous zincnitrite solution of the present invention, an aqueous solution with adesired zinc nitrite can be obtained. In the present invention, when thesulfate ion concentration as an impurity in the solution in terms of azinc nitrite solution [Zn(NO₂)₂] having an NO₂ concentration of 10% byweight is larger than 20 ppm, the remaining sulfate ions may be removedand purified if desired.

The purifying process may include, for example:

(1) a method which includes adding barium ions to the solution toprecipitate the sulfate ions as barium sulfate;

(2) a method which includes passing the solution through acation-exchange resin or an anion-exchange resin; and

(3) a method which includes a solvent extraction procedure. Method (1)is Preferred.

More specifically, it is sufficient to add a slight excess of bariumions relative to the residual sulfate ions and the addition amountrelative to the residual sulfate ions may be, for example 1.05 to 1.5equivalents, preferably 1.05 to 1.2 equivalents.

The aqueous zinc nitrite solution of the present invention can bepreferably used as a corrosion inhibitor, in particular, a film-formingpromoter for metal surface treatment or compositions thereof. Thefilm-forming promoter is an ingredient that forms a film on the surfaceof a metal by being added into a film-forming solution.

The film formed at this time may be, for example, a zinc phosphate film,an iron phosphate film, or a manganese phosphate film. When using theaqueous zinc nitrite solution of the present invention as a film-formingpromoter, zinc phosphate film is particularly preferred. When theaqueous zinc nitrite solution is used as the zinc phosphate film, in thetreating bath for forming the zinc phosphate film, the nitrite ions inthe zinc nitrite have promoting effects similar to that of the nitriteions in sodium nitrite. In addition, zinc ions are a major component ofzinc phosphate film, so that the both anions and cations in zinc nitritecan exhibit their effect as surface treating agents.

In addition, the aqueous zinc nitrite solution of the present inventionis reduced in the concentration of sodium ions and is substantially freeof sulfate ions and calcium ions in particular. The generation of sludgecan be reduced when the aqueous zinc nitrite solution is used as it isas a film-forming promoter for the metal surface treatment, or theaqueous zinc nitrite solution is used in combination with theconventional film-forming promoter. In addition, when a closed system ofmetal surface treatment is aimed at, an extremely efficient surfacetreatment on metal can be expected.

Other film-forming promoters for the metal surface treatment to be usedin combination are not particularly restricted but may include, forexample, nitrous acid, sodium nitrite, nitrite ammonium, m-nitrobenzenesulfonic acid sodium, a hydrogen peroxide, sodium chlorate, chlorateammonium, nitric acid, soda nitrate, ammonium nitrate, zinc nitrate,nitrate manganese, cobalt nitrate, calcium nitrate, nitrate magnesium,copper nitrate, and hydroxy amine. These are used alone or incombination with others.

When the aqueous zinc nitrite solution is used as a film-formingpromoter, in the range without affecting the characteristics of themetal surface treatment film and the characteristics of the metalsurface treatment film-forming agent, a stabilizing agent may be addedto stabilize the composition of the aqueous zinc nitrite solution byinhibiting the precipitation of crystals even at lower temperatures.

Such a stabilizing agent may include, for example, an alkali nitritemetal salt, sugar, or a chelating agent.

For the alkali nitrite metal salt, sodium nitrite, lithium nitrite, orthe like may be given.

For the sugar, sucrose, glucose, reduction malt sugar, mannitol,xylitol, starch, or the like may be given.

For the chelating agent, tartaric acid, gluconic acid, glycolic acid,glucuronic acid, ascorbic acid, citric acid, malic acid, ethylenediamine tetra-acetic acid, nitrilo triacetic acid, or glycine are given.

The above-mentioned stabilizing agents can be used alone or incombination with others. The addition amount of the stabilizing agent isusually 0.01 to 2% by weight with respect to the aqueous zinc nitritesolution.

EXAMPLES

The following examples illustrate the present invention in furtherdetail, but the present invention is not limited to these examples.

Example 1

Using a 5-cell electrodialyzer with ion-exchange membranes asillustrated in the figure, only NO₂ ions and Zn ions were transferredfrom an anion-exchange membrane (Selemion AMV; product of Asahi GlassCo.) and a cation-exchange membrane (Selemion CMV, product of AsahiGlass Co.) to obtain an aqueous zinc nitrite solution. The experimentalprocess is as follows.

Dissolved in deionized water were 575 g of zinc sulfate 7·H₂O to preparea 15% by weight aqueous solution of ZnSO₄ and this solution was fed tothe desalting chamber (I). On the other hand, 600 g of sodium nitritewere dissolved in deionized water to prepare a 30% by weight aqueoussolution of NaNO₂ and this solution was fed to the desalting chamber(III).

A 1.70% by weight aqueous solution of zinc nitrite was placed in theconcentrating chamber (II). The anode chamber and the cathode chamberwere supplied with a 3.0% by weight aqueous solution of Na₂SO₄.Anion-exchange membranes (A1, A2) and cation-exchange membranes (C1, C2)each having an effective membrane area of about 120 cm² were alternatelyarranged as shown in the figure. While the solutions were circulatedwith respective pumps to maintain uniformity of the concentration of thesolution in each chamber, a voltage of 5 V was applied to theion-exchange membranes to carry out an ion-exchange double decompositionreaction for 40 hours, whereby an aqueous solution of zinc nitrite wasobtained. Note that, in the resulting aqueous solution of zinc nitrite[Zn(NO₂)₂], the concentration of zinc nitrite was 17.7% by weight and,assuming that the concentration of said aqueous zinc nitrite solution tobe 10% by weight as NO₂, the sodium ion amount was 1188 ppm, the sulfateion amount was 10 ppm, and the calcium ion amount was 1 ppm.

The relationship between the concentration of Zn ions and theconcentration of NO₂ ions in the aqueous zinc nitrite solution of Theconcentration chamber (II) at dialysis time and also the relationshipamong the contents of sodium ions, sulfate ions, and calcium ions areshown in Table 1.

TABLE 1 Concentration chamber (II) Concen- Concen- Concen- Concen-Concen- Concen- tration tration tration tration tration tration Mole ofCurrent of Zn of NO₂ of Na of Ca of SO₄ Ratio Zn (NO₂)₂ Time ions ionsions ions ions NO₂/Zn ions (hr) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (wt%) 0 7444 9883 20 N.D. 0.7 1.89 1.7 1 10636 15191 117 N.D. 2.4 2.03 2.510 34018 48000 448 N.D. 4.8 2.00 7.7 19 54875 76976 693 N.D. 7.4 1.9912.0 29 71118 102265 954 N.D. 7.6 2.04 15.4 40 84299 119308 1188 N.D. 102.01 17.7 (Note) N.D. in the table indicates a detection limit of 1 ppmor less.

Examples 2 to 5

The concentration of zinc nitrite prepared by Example 1 was 17% byweight, and the stabilizing agents listed below were added, whileassuming that the concentration of the aqueous zinc nitrite [Zn(NO₂)₂]solution to be 10% by weight as NO₂, the sodium ion amount was 1188 ppm,the sulfate ion amount was 10 ppm, and the calcium ion amount was 1 ppm,followed by being sealed and left for 30 days at 5° C. in arefrigerator. After standing, the presence or absence of precipitate wasconfirmed by visual observation.

In the table, “absence” means that there was no precipitate.

Also, in Table 2, “Brending amount” indicates the addition amount withrespect to the aqueous zinc nitrite solution.

TABLE 2 Stabilizing agent Blending amount Presence or absence of Kind(wt %) precipitate Example 2 Sodium nitrite 2 Absence Example 3 Starch 1Absence Example 4 Citric acid 0.5 Absence Example 5 EDTA 0.1 Absence(Note) ″EDTA″ in Table 2 represents ethylene diamine tetra-acetic acid.

INDUSTRIAL APPLICABILITY

As described above, the aqueous zinc nitrite solution of the presentinvention is characterized by having a reduced concentration of sodiumions, and particularly being substantially free of sulfate ions andcalcium ions in particular. The aqueous zinc nitrite solution is used asa film-forming promoter for metal surface treatment or the compositionthereof. For instance, in the case of forming a zinc phosphate film onthe surface of steel or zinc by being added in a surface-treating agentof a zinc-phosphate system, the accumulation of ion impurities in atreating bath is small, the generation of sludge is also decreased, andthe frequency of replacing the liquid is greatly decreased. Also, when aclosed system is aimed at, an extremely efficient metal surfacetreatment can be expected.

Also, according the method for preparing an aqueous zinc nitritesolution, the aqueous zinc nitrite solution can be prepared by themethod having an extreme industrial advantage.

1. An aqueous zinc nitrite solution which contains 100 ppm or less ofcalcium (Ca) ions, characterized in that, in terms of the aqueous zincnitrite solution having an NO₂ concentration of 10% by weight, sodium(Na) ion concentration is 200 to 2000 ppm and sulfate (SO₄) ionconcentration is 20 ppm or less in the solution.
 2. An aqueous zincnitrite solution according to claim 1, further comprising a stabilizingagent.
 3. An aqueous zinc nitrite solution according to claim 2, whereinthe stabilizing agent is one selected from the group consisting ofalkali nitrite metal salts, sugars, and chelating agents.
 4. A methodfor preparing an aqueous zinc nitrite solution which contains 100 ppm orless of calcium (Ca) ions, characterized in that, in terms of theaqueous zinc nitrite solution having an NO₂ concentration of 10% byweight, sodium (Na) ion concentration is 200 to 2000 ppm and sulfate(SO₄) ion concentration is 20 ppm or less in the solution, comprisingsynthesizing a zinc compound and an alkali metal nitrite as rawmaterials by a double decomposition reaction using an ion-changemembrane as a diaphragm; and obtaining said aqueous zinc nitritesolution from said raw materials.
 5. A method for preparing an aqueouszinc nitrite solution according to claim 4, characterized in that thereaction is performed in an electrodialysis cell having a unit includingone concentration chamber and two desalting chambers sandwiching theconcentration chamber which are formed by an alternate arrangement ofcation-exchange membranes and anion-exchange membranes between an anodeand a cathode.
 6. A method for preparing an aqueous zinc nitritesolution according to claim 5, characterized in that an aqueous zinccompound solution is supplied to one of the desalting chambers and anaqueous alkali metal nitrite solution is supplied to the other of thedesalting chambers, and zinc ions are introduced through acation-exchange membrane and nitrite ions are introduced through ananion-exchange membrane into the concentration chamber sandwichedbetween the desalting chambers, to thereby obtain an aqueous zincnitrite solution.
 7. A method for preparing an aqueous zinc nitritesolution according to claim 4, characterized in that the zinc compoundis zinc sulfate and the alkali metal nitrite is sodium nitrite.